A vulnerability to viral infections may explain why babies treated with antibiotics are at an increased risk of developing asthma, research has shown.

Scientists found that infants given the drugs before their first birthday were more than twice as likely as untreated children to experience severe asthma symptoms at age 11.

Previous research has also linked early antibiotic treatment with asthma. But the new findings suggest it is not the drugs themselves but an immune system weakness that accounts for the link.

In the new study, the 11-year-olds with asthma were lacking in cytokines - signalling molecules which are the body's key defence against viral infections such as the common cold.

Two specific gene variants in a chromosomal region known as 17q21 also marked out these children as being more likely to be prescribed antibiotics as infants.

Lead scientist Professor Adnan Custovic, from the University of Manchester, said: "We speculate that hidden factors which increase the likelihood of both antibiotic prescription in early life and subsequent asthma are an increased susceptibility to viral infections due to impaired antiviral immunity and genetic variants on 17q21.

"However, further studies will be needed to confirm that the impaired immunity was present at the time of the early childhood respiratory symptoms and pre-dated antibiotic prescribing rather than as a consequence of the antibiotics."

The UK researchers, whose findings appear in The Lancet Respiratory Medicine journal, looked at data about 1,000 children studied from birth to age 11.

All were participants in the Manchester Asthma and Allergy Study (MAAS) investigating possible early triggers of allergy conditions.

At age 11, blood samples were tested to compare immune system responses to viruses and bacteria in children who had or had not been treated with antibiotics in their first year.

The results found that children with wheezing who were given antibiotics as infants were more than twice as likely as untreated children to experience worsening asthma symptoms, or severe wheeze, or be hospitalised for asthma.

They also pointed to an association with viral infections rather than bacteria.

Lung specialist Professor Seif Shaheen, from the Blizard Institute, Queen Mary, University of London, said: "An association between antibiotic use in early life and later childhood asthma has been found in many epidemiological studies.

"In recent years experts have concluded that most, if not all, of this link is likely to be explained by a tendency for young children with wheezing and respiratory infections (the commonest trigger of wheezing) to be treated with antibiotics, and for their wheezing to persist and become asthma later in childhood. In other words antibiotic use per se was not causing asthma to develop.

"This paper backs up this idea by showing that some children may be more susceptible to getting viral respiratory infections, and hence (inappropriate) antibiotic treatment, and also to developing asthma, because of low immune defences against viruses and because they have a particular genetic make-up.

"However, the measurements of the children's immune system in this study were made at 11 years of age. Therefore we cannot be certain that altered immunity was already present very early in life and influenced whether the children were given antibiotics."